The invention provides for methods for removing contaminants selected from the group consisting of nitrogen oxides, sulfur oxides, particulates, heavy metals and acid gases from gas streams.
More particularly, the invention provides for nitrogen oxides oxidation within a semi-dry scrubbing vessel without affecting the removal efficiencies for other contaminants at lower ozone consumption and further causing no issues of residual ozone or corrosion to downstream metallurgy of the semi-dry scrubber.
Semi-dry scrubbing has gained considerable success in lowering emissions from industrial gas exhaust systems such as those from metal roasting furnaces, sintering and pelletizing processes, lime and cement kilns where cleaning the exhaust stream in a wet scrubber is not a preferred option.
Nitrogen oxides from industrial sources mainly consist of NO which is not soluble and non-reactive. Nitrogen oxides are not removed in a dry or semi-dry scrubbing apparatus by commonly sued sorbents in the industry.
The methods of this invention uses the chemistry and techniques for oxidizing nitrogen oxides with ozone as described in U.S. Pat. Nos. 5,206,002; 6,162,409; 6,649,132; and 7,303,735 as well as US patent application publication 20140127107 in a dry or semi-dry scrubber.
Combustion and chemical processes generally result in gas streams containing contaminants that need to be cleaned up before the gas stream is exhausted to the atmosphere.
Many industrial processes, power generating utilities, combustion sources, stationary and mobile sources such as engines, boilers, kilns use solid fuels or low cost hydrocarbon fuels that contain sulfur, chlorine, nitrogen, and metal compounds in hydrocarbons which results in exhaust gases with contaminants such as acid gases, particulate matter and heavy metals. To comply with stricter environmental rules mandated by legislation, combinations of scrubbing (wet or dry) and particulate capture devices such as electrostatic precipitators (ESPs), wet ESPs and bag houses are increasingly preferred for emissions control of acid gas and particulate matters.
Some industrial segments prefer dry or semi-dry scrubbing. The most common semi-dry scrubbers for sulfur oxides (SOx) and acid gases are of two types. The first type uses scrubbing of contaminants by large size sorbent aggregates being contacted in the very turbulent zone of the scrubber vessel. Contaminants come into contact with the large aggregates and are adsorbed on the surface. Aggregates that are carried with the gas stream are separated and recycled and may be partly purged. The rejuvenation of the aggregates is done in the scrubber vessel itself or in the aggregates recycle stream.
The second type of semi-dry scrubber uses atomized or very small size aqueous spray of lime slurry in the scrubber vessel. It is often named a spray dryer. Successful scrubbers often employ atomized lime spray and exhaust gas flow in a co-current fashion.
Mercury may also be removed from the flue gas stream to be treated by the injection of sorbents such as activated carbon.
The temperature of the exhaust streams entering into the dry or semi-dry scrubbers are often in excess of 325° F. (162.7° C.). Injecting ozone upstream of the semi-dry scrubber causes significant loss of performance due to a variety of reasons. Both ozone and the oxidized form of nitrogen oxides are very unstable at a temperature that is in excess of 325° F. (162.7° C.). The excess use of ozone can compensate for the decomposition of both ozone and oxidized nitrogen oxides can be costly and the excess ozone can have unintended consequences in adsorbing other contaminants on the sorbent.
Injecting ozone downstream of a semi-dry scrubber is a better option than injecting ozone upstream of the scrubber. The temperature of the exhaust stream exiting the semi-dry scrubber is moderate enough for ozone to oxidize nitrogen oxides effectively and most of the contaminants have already been removed in the semi-dry scrubber leaving ozone little or no role to play in adversely affecting removal efficiencies.
In order to meet high removal efficiencies, substantial amounts of nitrogen oxides must be in the pentavalent form to react with sorbent in the gas stream leaving the semi-dry scrubber vessel. The reactions that lead nitrogen oxides oxidation to pentavalent form are many and the chemistry is complicated but for the sake of brevity they are simplified as:NO+O3→NO2+O2(very fast)  (1)NO2+O3→NO3+O2(slow)  (2)NO2+NO3→N2O5  (3)
In order to oxidize substantial portions of nitrogen oxides to the pentavalent form in a limited reaction time available in the duct between the semi-dry scrubber and the bag house, an excess amount of ozone is required. A small amount of unreacted ozone is very likely to remain in the gas stream that enters the bag house. This residual amount of ozone causes loss of the mechanical strength of most commonly used fabric filters in the bag house. Residual ozone going out of the stack is also not recommended. In addition, metallurgy of ducts downstream of the scrubber, bag house and exhaust may also be adversely affected due to the presence of oxy acids of oxidized nitrogen oxides and residual ozone.
Since nitrogen oxides found in most combustion exhaust streams is in the form of NO which is almost non-reactive, NO is not removed in dry or semi-dry scrubbers. Therefore, for controlling nitrogen oxides emissions with dry and semi-dry scrubbers, the two major options are i) lowering nitrogen oxides formation at the source by modifying combustion and ii) treating nitrogen oxides in the exhaust gas stream using post combustion techniques.
The primary techniques used for reducing nitrogen oxides formation by modifying combustion are the use of a low nitrogen oxides burner (LNB), flue gas recirculation (FGR), staged combustion and over fire air (OFA). Often these techniques are not adequate and sometimes not suitable and post combustion techniques such as selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR) become necessary to attain mandatory nitrogen oxides reduction.
Both SNCR and SCR have good success in addressing nitrogen oxides problems but they also both have limitations. Ozone based oxidation technologies have recently gained success as post combustion technique especially when an application is not suitable for SCR where exhaust is treated in a wet scrubber. Ozone based processes are described in U.S. Pat. Nos. 5,206,002; 6,162,409; and 7,303,735 (of common assignment herewith) providing multi-pollutant removal approach and they have been implemented on flue gas arising from gas and coal fired boilers removing multiple pollutants including nitrogen oxides, sulfur oxide, particulates, etc. Ozone based processes are also industrially practiced in lower emissions in many applications, namely, metal pickling processes, fluidized catalytic cracker (FCC) regenerator processes, metal recovery furnaces and sulfuric acid manufacture. These successes for ozone based processes at commercial scale have been limited to wet scrubbing.
The present invention is able to overcome these limitations and provide effective methods for removing nitrogen oxides by oxidizing with ozone and removing in a semi-dry scrubber.